In general, a color cathode-ray tube includes a vacuum envelope which has a substantially rectangular face panel having a curved effective portion, and a funnel which has a cylindrical neck portion at its one end and is joined together with the face panel. Further, a substantially rectangular shadow mask is arranged on the inner side of the phosphor screen which is formed on the inner surface of the effective portion and includes three-color phosphor layers.
An electron gun is arranged in the neck portion of the funnel. Three electron beams emitted from the electron gun are deflected by a magnetic field generated from a deflector (deflection yoke) mounted outside the funnel, and horizontally and vertically scan the phosphor screen through the shadow mask, and thereby displaying a color image.
The shadow mask includes a substantially rectangular mask body which is formed with a number of electron beam passing apertures at a surface facing the phosphor screen and has a plate thickness of about 0.1 to 0.3 mm, and a substantially rectangular mask frame which is fixed to an outer peripheral portion of the mask body. In general, the surface of the mask body on which the electron beam passing apertures are formed is constructed as a curved surface such that at least central portion thereof projects toward the phosphor screen so as to correspond to the shape of the inner surface of the effective portion of the face panel. As a shape of the curved surface, in the prior art, there are a spherical surface, a cylindrically curved surface which has a radius of curvature in a short axis direction thereof being approximately infinity and is curved in a long axis direction thereof, a curved surface represented by a high-order polynomial expression, etc.
Meanwhile, the methods of forming the curved surface of the shadow mask include a press forming method and a method of forming a curved surface by applying a tension.
The press forming method is a method of plastically deforming a flat plate like shadow mask made of a metallic thin plate having a number of electron beam passing apertures into a desired shape by press working. This method is employed mainly to form a shadow mask having a spherical surface or a curved surface represented by a high-order polynomial expression.
The method of forming a curved surface by applying a tension is employed mainly to form a cylindrically curved surface which has a radius of curvature in a short axis direction thereof being approximately infinity and is curved in a long axis direction thereof. According to this method, a flat plate like shadow mask, which is made of a metallic thin plate having a number of electron beam passing apertures formed thereon, is curved so as to face a frame having a mask body attachment surface which has a radius of curvature of the short axis direction being approximately infinity and is curved in the long axis direction, and then, the flat plate like shadow mask is fixed to the frame in a state that a tension is applied in the short axis direction, and thereby, a shadow mask having a desired shape being formed.
Recently, in a color cathode-ray tube, the effective portion of the face panel is made flat in order to improve a visibility of the screen. With the flatness of the face panel, the curved surface of the shadow mask is made flat, and also, the curvature thereof is becoming smaller.
In the case where the shadow mask which is made flat is formed by the aforesaid press forming, since the curvature thereof is small, a curved surface holding strength of the shadow mask becomes low. As a result, if an external force such as impact or the like is applied to the color cathode-ray tube, the curved surface portion of the shadow mask is liable to be deformed. Further, if a vibration is applied to the color cathode-ray tube, the shadow mask is liable to be resonated (howling). This causes a deterioration in a color purity of a displayed image.
Also, in the case of locally displaying a high luminance image, the shadow mask is locally heated by a collision of high-density electron beams, and is locally bulged into the phosphor screen direction; namely, a so-called doming is liable to occur. If the aforesaid doming occurs, in particular, the electron beams do not properly land at a predetermined phosphor layers on an intermediate area of the screen, causing a deterioration in a color purity, likewise above.
The curved surface holding strength of the flattened shadow mask can be improved by making thick a plate thickness of the shadow mask. If the plate thickness of the shadow mask is thickened, however, not only it is difficult to form a electron beam passing apertures having a desired shape and dimension, but also a material cost increases.
On the other hand, the method of forming a curved surface by applying a tension is a method of applying a tensile stress to the shadow mask by applying a tension to a short axis direction in which a radius of curvature is approximately infinity. For this reason, a radius of curvature of a long axis direction is made large, and the curved surface of the shadow mask can be easily flattened. Further, a predetermined curved surface holding strength can be given to the shadow mask.
According to this method, however, an extremely great tensile stress must be applied to the shadow mask; for this reason, an extremely strong frame for holding the shadow mask is required. Consequently, with the color cathode-ray tube being large-sized, a weight of the frame is greatly increased.
In order to solve the above problems, there has been proposed a method of forming the curved surface of the shadow mask into a substantially cylindrically curved shape which has a radius of curvature in the long axis direction being approximately infinity and is curved in the short axis direction or a shape represented by a high-order polynomial expression. Further, in order to manufacture the aforesaid shadow mask, there has been proposed a method comprising the steps of bending a flat plate shadow mask made of a. metallic thin plate so as to have a curvature in only short axis direction, plastically deforming the shadow mask into a cylindrically curved shape (circular arc), elastically deforming the shadow mask so that a predetermined curved surface having a radius of curvature larger than the above curvature, and fixing the shadow mask to the frame.
According to this method, an elastic force of making small the radius of curvature, that is, a stress of improving a curved surface holding strength is applied to the shadow mask. Therefore, the curved surface holding strength of the shadow mask can be improved.
However, the shadow mask as described above has a problem that the curved surface holding strength of the shadow mask is low at the portions which are close to the long axis of the shadow mask if the shadow mask is made a large scale accompanying with a large scale of a color cathode-ray tube.